The present invention relates to an optical spot scanning system and more particularly to an improved holographic scanning system which includes a compensation element to correct for cross-scan errors in the scan line due to wavelength shifts occurring in the light source.
Holographic scanners which utilize a rotating disc having a plurality of holographically formed lenses or gratings are known in the art. Representative disclosures are provided in the Prior Art List filed with the present application. These prior art spinners are subject to certain problems resulting from their geometry. These problems, briefly stated, are scan line "bow", in the image plane, spinner wobble and spinner "wedge" both resulting in colinear multiple scan lines, and spinner decentration causing output scan distortion. These problems are more thoroughly analyzed in co-pending U.S. Application Ser. No. 044,000 filed on May 31, 1979 now U.S. Pat. No. 4,289,371 and assigned to the same assignee as the present invention. In this co-pending application, whose contents are hereby incorporated by reference, the enumerated problems are compensated for by utilizing, as the reconstruction element, a spinner having on its surface a plurality of holographically formed plane linear diffraction gratings. By strict mathematical methods, it was demonstrated that certain relationships existed between the wavelength of the reconstruction light source, the grating period and the angles of incidence and diffraction, whereby most of the inherent spinner problems were either corrected or minimized. The application also addressed another problem, wavelength shift, which originates with the reconstruction light source. This shift, or change in the source wavelength, results in corresponding changes in the output diffraction angle, and hence, an undesirable deflection of the output scan line. The plane linear diffraction grating spinner is effective only when a monochromatic, stable-wavelength, light source was utilized. Thus, a stable source He-Ne laser was used in the exemplary example provided in said application.
It is therefore desirable to utilize a plane grating holographic spinner such as that described in the co-pending application but which is further improved by elimination of the effects caused by wavelength shifts in the coherent light source. Such a technique is becoming of considerable importance because of the increasing use of laser diodes as the light source in scanning systems. These diodes can experience wavelength shifts of up to 3 nm or more due to junction heating over its output power range.
The present invention is therefore directed to an optical scanning system including a spinner having formed thereon at least one plane linear diffraction grating having a constant grating period d, a stationary wavelength compensation diffraction grating having properties identical to said grating formed on said spinner surface, said stationary grating placed in a plane parallel and in optical alignment with said spinner grating, a collimated reconstruction light source of wavelength .lambda..sub.r which provides a beam of light directed at an angle of incidence .theta..sub.i onto said compensation grating, said grating diffracting said beam at a diffraction angle of .theta..sub.d, said incidence angle .theta..sub.i .perspectiveto..theta..sub.d .perspectiveto.45.degree., and the ratio of .lambda..sub.r to grating spacing d having a value between 1 and 1.618, whereby the light beam diffracted by the compensation grating is incident on the spinner grating at an angle .phi..sub.i and is diffracted out of said grating at an angle .phi..sub.d .perspectiveto..theta..sub.i.